We discuss two procedures to obtain empirical potentials from ab initio trajectories. The first methodconsists in adjusting the parameters of an empirical pair potential so that the radial distribution functionsextracted from classical simulations using this potential match the ones extracted from the ab initio sim-ulations. As a case study, we consider the example of amorphous silica, a material that is highly relevantin the field of glass science as well as in geology. With our approach we are able to obtain an empiricalpotential that gives a better description with respect to structural and thermodynamic properties thanthe potential proposed by van Beest, Kramer, and van Santen, and that has been very frequently usedas a model for amorphous silica. The second method is the so-called ‘‘force matching” approach proposedby Ercolessi and Adams to obtain an empirical potential. We demonstrate that for the case of silica thismethod does not yield a reliable potential and discuss the likely origin for this failure.

Nanoindentation of the pristine and irradiated forms of a sodium borosilicate glass: Insights from molecular dynamics simulations

We have carried out classical molecular dynamics simulations in order to get insight into the atomistic mechanisms of the deformation during nanoindentation of the pristine and irradiated forms of a sodium borosilicate glass. In terms of the glass hardness, we have found that the primary factor affecting the decrease of hardness after irradiation is depolymerization rather than free volume, and we argue that this is a general trend applicable to other borosilicate glasses with similar compositions. We have analyzed the changes of the short- and medium-range structures under deformation and found that the creation of oxygen triclusters is an important mechanism in order to describe the deformation of highly polymerized borosilicate glasses and is essential in the understanding of the folding of large rings under stress. We have equally found that the less polymerized glasses present a higher amount of relative densification, while the analysis of bond-breaking during the nanoindentation has showed that shear flow is more likely to appear around sodium atoms. The results provided in this study can be proven to be useful in the interpretation of experimental results.

We have carried out Molecular Dynamics simulations on a sodium borosilicate glass in order to analyze how the structure of the glass during irradiation is affected by the choice of the density in the liquid state before cooling. In a pristine form generated through the usual melt-and-quench method, both short- and medium-range structures are affected by the compressive or tensile environment under which the glass model has been generated. Furthermore, Na-rich areas are much easier to compress, producing a more homogeneous glass, in terms of density, as we increase the confinement during the quench. When the glass is subjected to displacement cascades, the structural modifications saturate at a deposited energy of approximately 8 eV/atom. Swelling appears for the glasses that were initially prepared under compression, while contraction is evident for the ones prepared under tension. We have equally prepared glass models using a fast quench method, and we have found that they present an analogous disorder as the glasses submitted to displacement cascades. Compared to the irradiated glass, we found that the magnitude of the modifications for the fast quenched glass is lower, most notably in terms of boron and sodium coordination, the percentage of non-bridging oxygens and in the ring distributions. This later result agrees with statements extracted from recent experimental works on nuclear glasses.